Any electrical circuit that uses a transistor as an amplifier needs to bias the transistor at some operating point so that the device is in the active region. The current through the device or the voltage applied to the transistor device will determine this operating or bias point. Over temperature, the bias point also needs to be maintained by a stable bias current, so that the design characteristics will also remain within the specified design limits.
Conventional circuits employing only bipolar transistors to generate the temperature stable bias current are generally well known in the prior art. Because the voltage V.sub.BE across the base-emitter junction of a bipolar transistor has a negative temperature coefficient, the currents derived from this voltage inevitably will also have the same negative characteristic. Due to higher density and lower power consumption, MOS is the technology of choice in today's IC circuits. However, the threshold voltage V.sub.T of a MOS transistor, which is the equivalent of V.sub.BE in bipolar technology, has a positive temperature coefficient from which to derive currents.
It would therefore be desirable to provide a merged or composite bipolar/MOS circuit which combines the advantages of bipolar and MOS technologies together. As a result bipolar transistors and MOSFET transistors are merged or are arranged in a common semi-conductor substrate in order to form an integrated circuit which can give a precise control of bias current over temperature and can be manufactured at a relatively low cost but yet provides a much improved performance.